1. Field of the Invention
This invention relates to a method for detecting the blending ratio of mixed fuel obtained by mixing different kinds of fuels when the mixed fuel is supplied to the combustion chamber of an internal combustion engine.
2. Description of the Related Art
Since the combustion gas produced when methanol is perfectly burned contains only a small amount of harmful substances, methanol has received much attention as a fuel for automobiles. That is, when methanol is used as a fuel for automobiles, it is an advantage that environmental pollution due to the exhaust gas of the automobiles can be prevented. Thus, methanol is excellent as a fuel for automobiles and therefore a methanol engine using methanol as the fuel has been developed.
However, it is practically impossible to instantly change the fuel of all automobiles from gasoline to methanol. Therefore, even in a case where the fuel of the automobile is completely changed from gasoline to methanol, there may be a situation in which gasoline is used as the fuel of the automobile as well as methanol in a transition period.
In such a situation in which different kinds of fuels are used at the same time as the fuel of the automobile, it is required to develop an engine which can use gasoline, methanol and mixed fuel obtained by mixing gasoline and methanol as the fuel thereof.
In such an engine, it is impossible that control variables such as an ignition timing of the mixed gas (mixture of the mixed fuel and air) in the combustion chamber and an amount of mixed fuel injected into the combustion chamber from the fuel injector exactly coincide with desired target values, respectively, unless the blending ratio of the mixed fuel to be injected into the combustion chamber, that is, the mixing ratio of gasoline and methanol, is correctly detected.
In order to detect the blending ratio of the mixed fuel, two detection methods are provided, for example. One of the detection methods is to indirectly detect the blending ratio of the mixed fuel by use of an oxygen sensor. The oxygen sensor is used to detect the concentration of oxygen contained in the exhaust gas of the engine. Therefore, an air/fuel ratio of the mixture supplied to the combustion chamber can be detected based on an output signal of the oxygen sensor. Thus, the blending ratio of the mixed fuel can be indirectly obtained by learning a blending ratio of the mixed fuel estimated from the value of the detected air/fuel ratio based on the feedback control.
However, the oxygen sensor has a property that the value of the output signal thereof cannot be correctly obtained until the operating condition of the engine becomes stable. For this reason, with the method using the oxygen sensor, the blending ratio of the mixed fuel cannot be detected on a real time basis.
On the other hand, the other detection method uses a blending ratio sensor and the blending ratio sensor can directly detect the blending ratio of the mixed fuel to be supplied to the combustion chamber. Therefore, the method using the blending ratio sensor is advantageous over the method using the oxygen sensor in that the blending ratio of the mixed fuel can be detected on a real time basis. However, the blending ratio sensor detects the blending ratio of the mixed fuel flowing through the blending ratio sensor on a real time basis, but does not detect the blending ratio of the mixed fuel injected from the fuel injector on a real time basis. Therefore, an error may occur between the blending ratio of the mixed fuel derived from the output signal of the blending ratio sensor and the blending ratio of the mixed fuel actually injected from the fuel injector.